How often does this happen? How many assets are affected? Does this happen for all assets after the mentioned few days? Is this happening only on author?
Can you check the filesystem where the datastore lives if everything is correct there?
Technically it could be a workflow, which modifies the image (or the renditions you view). Maybe you can check that; just given the high number of affected assets I would not think that it's a defect in the filesystem at the first place (but of course it could be).